I wanted to discuss a slightly hidden feature of SWMM 5 that has a definite impact on flat or links with zero slope. SWMM5 uses an internal unit system based on feet and cfs even though externally you can use SI units. There is a minimum conduit drop between the upstream and downstream ends of a SWMM 5 link which is 0.001 feet. This is done so that every link has a non zero slope. However, depending on the length of the link the drop of 0.001 feet results in drastically different slopes.
Here is a table of the length of link (feet) and the internal slope used in SWMM 5 based on the minimum drop of 0.001 feet. As you can see a very long link can have a very small slope which has time step issues - you need a smaller time step.
Here is a table of the length of link (feet) and the internal slope used in SWMM 5 based on the minimum drop of 0.001 feet. As you can see a very long link can have a very small slope which has time step issues - you need a smaller time step.
| Length | Slope |
| 0.1 | 1.0000000% |
| 0.5 | 0.2000000% |
| 1 | 0.1000000% |
| 2 | 0.0500000% |
| 3 | 0.0333333% |
| 4 | 0.0250000% |
| 5 | 0.0200000% |
| 6 | 0.0166667% |
| 7 | 0.0142857% |
| 8 | 0.0125000% |
| 9 | 0.0111111% |
| 10 | 0.0100000% |
| 25 | 0.0040000% |
| 50 | 0.0020000% |
| 75 | 0.0013333% |
| 100 | 0.0010000% |
| 200 | 0.0005000% |
| 300 | 0.0003333% |
| 400 | 0.0002500% |
| 500 | 0.0002000% |
| 600 | 0.0001667% |
| 700 | 0.0001429% |
| 800 | 0.0001250% |
| 900 | 0.0001111% |
| 1000 | 0.0001000% |
| 2000 | 0.0000500% |
| 5000 | 0.0000200% |
| 10000 | 0.0000100% |
